Khan Academy

Khan Academy is one of the first successful free online education.
It is a production of Salman Khan (1976); the American son of Bangladeshi (father) and indian (mother) immigrants.
I highly recommend to get use of this rich site (all family members), and to share their highly appreciated efforts.
Subjects and exercises could serve directly individuals study materials, or could be a useful continuous mind training.

Today, Khan Academy celebrated, with its 13 million users, 2 billion answered math problems, aiming to reach 10 billions in the coming year!